I can't image the 2 series coupes & verts will be 6 cylinders unless the M2 is much lighter and turbo'ed out, thusly making it much faster. Even if this were the case, people would be very hesitant to pay more for a car with fewer cylinders, even if the output numbers are there.
The original 2002 had a two-liter four cylinder engine and was very successful due to light weight and good design. One of the complaints many had about the current 1-series when it debuted was that it didn't weigh much less than a 3-series. In order to lose any meaningful amount of weight, the option of an inline six would need to be dropped.
If the weight drops and the new generation of turbo fours make ~300 bhp or so, the car will be faster and handle better than the current generation. My Lotus Elise does just fine with four cylinders due to keeping the weight down. The current 1-series feels like a truck in comparison.
Another factor that will drive weight reduction and smaller engines is fuel consumption. The new CAFE requirements and continued increases in fuel cost will make cars that get EPA ratings in the teens less desirable in the future. If a four cylinder 2 series can match or beat straight line performance of the current 1 series, offer better handling and better fuel economy, I would think that people would "pay for a car with fewer cylinders". I know that I would.
'11 BMW 335i (Alpine White) M-Sport, Premium, Heated Seats, HK, Ipod adaptor, dealer-installed PPK - Euro Delivery 5/11/2011
'05 Lotus Elise (Arctic Silver) Sport, HT, Stage II, Odyssey PC925
'87 Nissan Pickup (Burgandy) King Cab, 80k miles
'13 Audi A3 TDI (Ibis White) Cold Weather Package, Bluetooth - Wife's car - Delivery 9/12